A bolt carrier group (BCG) 100 of a traditional AR15-type firearm, as shown in FIG. 13, usually consists of a bolt carrier 103, a bolt assembly 101, a firing pin 105, a cam pin 102, and a firing pin retaining pin 104. For a direct impingement type of gas system to operate, hot gas directed from a gas tube (not shown) of a firearm would flow into the bolt carrier 103 through a gas channel 106 on the bolt carrier 103 and into the chamber (not shown) inside the bolt carrier 103 and push the bolt assembly 101 to rotate under a force generated by the cam pin 102. The hot gas typically has a mild pressure compared to the pressure in the gun barrel chamber; however, it is still considered as high pressure compared to ambient air pressure. A length 107 is the dimension between a front face of the bolt carrier 103 and a full auto actuation surface 108 on a rear face of the bolt carrier 103.
Referring to FIG. 14-FIG. 16, a lower assembly of a traditional AR15 firearm consists of a lower receiver 201, a buffer 202, a main spring 203, an end plate 204, a castle nut 205, a buffer tube 206, and a stock 207. The buffer tube 206 houses the buffer 202 and main spring 203 therein, while the castle nut 205 fastens the end plate 204 and the buffer tube 206 to the lower receiver 201. Once the firearm fires one cartridge, the BCG would retract back toward the back of the firearm and compress the main spring 203. A tail portion of the bolt carrier 103 would sink into or otherwise be received in the buffer tube 206. The stock 207 is attached to the buffer tube 206 though its own latching mechanism (not shown), such that the stock 207 can slide forward and backward as well as fasten to the buffer tube 206 at various points on the buffer tube 206, so that adjustment can be made to satisfy a specific length of arms of a user to allow the user to operate the firearm comfortably.
Current designs of AR15 firearms demand a buffer tube in various lengths greater than 7 to 8 inches in order for the direct impingement type of gas system to function properly. As such, the buffer tube 206, and the stock that 207 that is attached to it, cannot be folded to one side of the firearm to shorten the weapon system for carrying by a user in various compact situations, some of which would demand the weapon to be packed as short as possible.
Presently there are some folding stock adaptor designs that can fold the buffer tube and stock of a firearm with a direct impingement type of gas system. However, with such designs, when the buffer tube/stock is folded it is not safe to fire the firearm, thereby rendering the firearm useless after firing a first round of cartridge. In order for such designs to function, the buffer tube/stock needs to be unfolded to be in a combat configuration for the system to operate properly.
There exist other designs for shortening the buffer tube and the bolt carrier. However, those designs tend to render disassembly of the firearm difficult.